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Safe(3p)	Perl Programmers Reference Guide	 Safe(3p)

NAME
     Safe - Compile and execute code in restricted compartments

SYNOPSIS
       use Safe;

       $compartment = new Safe;

       $compartment->permit(qw(time sort :browse));

       $result = $compartment->reval($unsafe_code);

DESCRIPTION
     The Safe extension module allows the creation of compart-
     ments in which perl code can be evaluated. Each compartment
     has

     a new namespace
	     The "root" of the namespace (i.e. "main::") is
	     changed to a different package and code evaluated in
	     the compartment cannot refer to variables outside
	     this namespace, even with run-time glob lookups and
	     other tricks.

	     Code which is compiled outside the compartment can
	     choose to place variables into (or share variables
	     with) the compartment's namespace and only that data
	     will be visible to code evaluated in the compart-
	     ment.

	     By default, the only variables shared with compart-
	     ments are the "underscore" variables $_ and @_ (and,
	     technically, the less frequently used %_, the _
	     filehandle and so on). This is because otherwise
	     perl operators which default to $_ will not work and
	     neither will the assignment of arguments to @_ on
	     subroutine entry.

     an operator mask
	     Each compartment has an associated "operator mask".
	     Recall that perl code is compiled into an internal
	     format before execution. Evaluating perl code (e.g.
	     via "eval" or "do 'file'") causes the code to be
	     compiled into an internal format and then, provided
	     there was no error in the compilation, executed.
	     Code evaluated in a compartment compiles subject to
	     the compartment's operator mask. Attempting to
	     evaluate code in a compartment which contains a
	     masked operator will cause the compilation to fail
	     with an error. The code will not be executed.

	     The default operator mask for a newly created

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Safe(3p)	Perl Programmers Reference Guide	 Safe(3p)

	     compartment is the ':default' optag.

	     It is important that you read the Opcode(3) module
	     documentation for more information, especially for
	     detailed definitions of opnames, optags and opsets.

	     Since it is only at the compilation stage that the
	     operator mask applies, controlled access to poten-
	     tially unsafe operations can be achieved by having a
	     handle to a wrapper subroutine (written outside the
	     compartment) placed into the compartment. For exam-
	     ple,

		 $cpt = new Safe;
		 sub wrapper {
		     # vet arguments and perform potentially unsafe operations
		 }
		 $cpt->share('&wrapper');

WARNING
     The authors make no warranty, implied or otherwise, about
     the suitability of this software for safety or security pur-
     poses.

     The authors shall not in any case be liable for special,
     incidental, consequential, indirect or other similar damages
     arising from the use of this software.

     Your mileage will vary. If in any doubt do not use it.

     RECENT CHANGES

     The interface to the Safe module has changed quite dramati-
     cally since version 1 (as supplied with Perl5.002). Study
     these pages carefully if you have code written to use Safe
     version 1 because you will need to makes changes.

     Methods in class Safe

     To create a new compartment, use

	 $cpt = new Safe;

     Optional argument is (NAMESPACE), where NAMESPACE is the
     root namespace to use for the compartment (defaults to
     "Safe::Root0", incremented for each new compartment).

     Note that version 1.00 of the Safe module supported a second
     optional parameter, MASK.	That functionality has been with-
     drawn pending deeper consideration. Use the permit and deny
     methods described below.

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Safe(3p)	Perl Programmers Reference Guide	 Safe(3p)

     The following methods can then be used on the compartment
     object returned by the above constructor. The object argu-
     ment is implicit in each case.

     permit (OP, ...)
	     Permit the listed operators to be used when compil-
	     ing code in the compartment (in addition to any
	     operators already permitted).

	     You can list opcodes by names, or use a tag name;
	     see "Predefined Opcode Tags" in Opcode.

     permit_only (OP, ...)
	     Permit only the listed operators to be used when
	     compiling code in the compartment (no other opera-
	     tors are permitted).

     deny (OP, ...)
	     Deny the listed operators from being used when com-
	     piling code in the compartment (other operators may
	     still be permitted).

     deny_only (OP, ...)
	     Deny only the listed operators from being used when
	     compiling code in the compartment (all other opera-
	     tors will be permitted).

     trap (OP, ...)
     untrap (OP, ...)
	     The trap and untrap methods are synonyms for deny
	     and permit respectfully.

     share (NAME, ...)
	     This shares the variable(s) in the argument list
	     with the compartment. This is almost identical to
	     exporting variables using the Exporter module.

	     Each NAME must be the name of a non-lexical vari-
	     able, typically with the leading type identifier
	     included. A bareword is treated as a function name.

	     Examples of legal names are '$foo' for a scalar,
	     '@foo' for an array, '%foo' for a hash, '&foo' or
	     'foo' for a subroutine and '*foo' for a glob (i.e.
	     all symbol table entries associated with "foo",
	     including scalar, array, hash, sub and filehandle).

	     Each NAME is assumed to be in the calling package.
	     See share_from for an alternative method (which
	     share uses).

     share_from (PACKAGE, ARRAYREF)

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	     This method is similar to share() but allows you to
	     explicitly name the package that symbols should be
	     shared from. The symbol names (including type char-
	     acters) are supplied as an array reference.

		 $safe->share_from('main', [ '$foo', '%bar', 'func' ]);

     varglob (VARNAME)
	     This returns a glob reference for the symbol table
	     entry of VARNAME in the package of the compartment.
	     VARNAME must be the name of a variable without any
	     leading type marker. For example,

		 $cpt = new Safe 'Root';
		 $Root::foo = "Hello world";
		 # Equivalent version which doesn't need to know $cpt's package name:
		 ${$cpt->varglob('foo')} = "Hello world";

     reval (STRING)
	     This evaluates STRING as perl code inside the com-
	     partment.

	     The code can only see the compartment's namespace
	     (as returned by the root method). The compartment's
	     root package appears to be the "main::" package to
	     the code inside the compartment.

	     Any attempt by the code in STRING to use an operator
	     which is not permitted by the compartment will cause
	     an error (at run-time of the main program but at
	     compile-time for the code in STRING).  The error is
	     of the form "'%s' trapped by operation mask...".

	     If an operation is trapped in this way, then the
	     code in STRING will not be executed. If such a
	     trapped operation occurs or any other compile-time
	     or return error, then $@ is set to the error mes-
	     sage, just as with an eval().

	     If there is no error, then the method returns the
	     value of the last expression evaluated, or a return
	     statement may be used, just as with subroutines and
	     eval(). The context (list or scalar) is determined
	     by the caller as usual.

	     This behaviour differs from the beta distribution of
	     the Safe extension where earlier versions of perl
	     made it hard to mimic the return behaviour of the
	     eval() command and the context was always scalar.

	     Some points to note:

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	     If the entereval op is permitted then the code can
	     use eval "..." to 'hide' code which might use denied
	     ops. This is not a major problem since when the code
	     tries to execute the eval it will fail because the
	     opmask is still in effect. However this technique
	     would allow clever, and possibly harmful, code to
	     'probe' the boundaries of what is possible.

	     Any string eval which is executed by code executing
	     in a compartment, or by code called from code exe-
	     cuting in a compartment, will be eval'd in the
	     namespace of the compartment. This is potentially a
	     serious problem.

	     Consider a function foo() in package pkg compiled
	     outside a compartment but shared with it. Assume the
	     compartment has a root package called 'Root'. If
	     foo() contains an eval statement like eval '$foo =
	     1' then, normally, $pkg::foo will be set to 1.  If
	     foo() is called from the compartment (by whatever
	     means) then instead of setting $pkg::foo, the eval
	     will actually set $Root::pkg::foo.

	     This can easily be demonstrated by using a module,
	     such as the Socket module, which uses eval "..." as
	     part of an AUTOLOAD function. You can 'use' the
	     module outside the compartment and share an (auto-
	     loaded) function with the compartment. If an auto-
	     load is triggered by code in the compartment, or by
	     any code anywhere that is called by any means from
	     the compartment, then the eval in the Socket
	     module's AUTOLOAD function happens in the namespace
	     of the compartment. Any variables created or used by
	     the eval'd code are now under the control of the
	     code in the compartment.

	     A similar effect applies to all runtime symbol look-
	     ups in code called from a compartment but not com-
	     piled within it.

     rdo (FILENAME)
	     This evaluates the contents of file FILENAME inside
	     the compartment. See above documentation on the
	     reval method for further details.

     root (NAMESPACE)
	     This method returns the name of the package that is
	     the root of the compartment's namespace.

	     Note that this behaviour differs from version 1.00
	     of the Safe module where the root module could be
	     used to change the namespace. That functionality has

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Safe(3p)	Perl Programmers Reference Guide	 Safe(3p)

	     been withdrawn pending deeper consideration.

     mask (MASK)
	     This is a get-or-set method for the compartment's
	     operator mask.

	     With no MASK argument present, it returns the
	     current operator mask of the compartment.

	     With the MASK argument present, it sets the operator
	     mask for the compartment (equivalent to calling the
	     deny_only method).

     Some Safety Issues

     This section is currently just an outline of some of the
     things code in a compartment might do (intentionally or
     unintentionally) which can have an effect outside the com-
     partment.

     Memory  Consuming all (or nearly all) available memory.

     CPU     Causing infinite loops etc.

     Snooping
	     Copying private information out of your system. Even
	     something as simple as your user name is of value to
	     others. Much useful information could be gleaned
	     from your environment variables for example.

     Signals Causing signals (especially SIGFPE and SIGALARM) to
	     affect your process.

	     Setting up a signal handler will need to be care-
	     fully considered and controlled.  What mask is in
	     effect when a signal handler gets called?	If a user
	     can get an imported function to get an exception and
	     call the user's signal handler, does that user's
	     restricted mask get re-instated before the handler
	     is called? Does an imported handler get called with
	     its original mask or the user's one?

     State Changes
	     Ops such as chdir obviously effect the process as a
	     whole and not just the code in the compartment. Ops
	     such as rand and srand have a similar but more sub-
	     tle effect.

     AUTHOR

     Originally designed and implemented by Malcolm Beattie,
     mbeattie@sable.ox.ac.uk.

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Safe(3p)	Perl Programmers Reference Guide	 Safe(3p)

     Reworked to use the Opcode module and other changes added by
     Tim Bunce <Tim.Bunce@ig.co.uk>.

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